We have already referred to the position of the mouth, which is situated between the mandibles.

In front w r e have a fleshy upper-lip, and below an under-lip, which in every way, both in structure and development, appears to be an appendage homologous with the others, but which, until the present decade, has never been so considered.

Respiration is effected (with a very few exceptions) by gills, which are always present, and are attached to the basal joints of the thoracic limbs and a few of the mouth- parts, and extend up into the gill-chamber already described.

The heart is a strong, small, and compact organ, situated in the middle of the upper part of the thorax.

From it arise five arteries ; one supplying the eyes, one the upper surface of the abdomen, one goes down to the lower surface of the body, and there dividing, supplies the appendages, while the two remaining arteries, one on each side, convey the blood from the heart to the viscera.